Boris Johnson has been referred to the police watchdog to assess whether he should be investigated for the criminal offence of misconduct in public office while he was mayor of London.
The Greater London Authority (GLA) said in a statement that the authority's monitoring officer had recorded a "conduct matter" against the Prime Minister over allegations the businesswoman Jennifer Arcuri received favourable treatment because of her friendship with him.
The Independent Office for Police Conduct (IOPC) is now looking into the claims.
"A 'conduct matter' exists where there is information that indicates that a criminal offence may have been committed," the statement said.
"It does not mean that this is proved in any way. The IOPC will now consider if it is necessary for the matter to be investigated."
The issue has been referred to the IOPC, which deals with complaints against police forces in England and Wales, because Johnson at the time held the post equivalent to a police and crime commissioner.
The PM, who denies any wrongdoing, has previously said that he will co-operate with any investigation into his links with Ms Arcuri by the London Assembly.
